It is known to convert glycerol to propylene glycol. Glycerol that is derived from a bio-diesel process has to be treated in a number of steps prior to its conversion to propylene glycol. For instance, one step includes the acidification of the glycerol feed to decant the free fatty acids. In addition, the glycerol is often thermally stripped to remove methanol. Thermally stripping the glycerol feed to remove methanol, however, has the adverse consequence of removing much of the water that is otherwise necessary to enable the catalytic process. Thus, concentrated glycerol feed must be diluted with water to about 40% to 60% by weight prior to its use as reactor feed. However, the necessary addition of water to the feed stream places a burden on the overall system since it must later be removed in downstream distillation separation processes. As such, having to remove water that was just added in previous steps is energy inefficient, and increases production time and the overall cost of the process.
Further, once the concentrated glycerol is diluted with water, a base reagent (such as sodium hydroxide) must be added to the glycerol before it can be used as reactor feed. Unfortunately, the base reagent that leaves with the reactor product is not easily recycled and is wasted during the recovery process. Finally, there is significant data on this process that suggests that in many cases the process operates near a hydrogen mass transport limited regime.
Accordingly, there exists a need for improved processes to convert glycerol to propylene glycol.